Peripheral interface adapter circuits have been developed to assist in interfacing a microcomputer with systems which are asynchronous with the microcomputer. One problem in this regard is providing a count of asynchronously received signals to the microcomputer. A read portion of a system cycle of the microcomputer may occur while a counter is updating its count of the asynchronous signals. Because an output of the counter may be invalid during the update, the count read by the microcomputer would be considered invalid.
Measures must be taken to ensure that the counter output is stable long enough before the read portion of the cycle is completed for the microcomputer to complete the reading of the counter contents. Because peripheral interface adapter circuits may be separated from the microcomputer by a significant distance, it is desirable to require as few signals from the microcomputer as possible to achieve synchronization. Consequently, only one synchronizing signal may be available.
The duration of time that the asynchronous signal is present to be counted may not be known. Consequently, it is important to be able to count independently of the duty cycle of the asynchronous signal.
The faster a counter must be, the larger the chip area required for the counter. The counter may be a ripple counter or a synchronous counter. Synchronous counters are faster than ripple counters by the nature of the design. Synchronous counters, however, require more devices and therefore more chip area. The speed of ripple counters is related to the speed of the devices from which it is made. Because faster devices require more chip area, the faster the ripple counter the more chip area required. It is therefore desirable to know how much time is available for the counter to perform its function so that the counter can so be designed to minimize the chip area occupied by the counter.
The frequency of the asynchronous signals to be counted may be unknown. A synchronizing circuit for handling such signals must be able to handle low as well as high frequencies The higher the frequency that can be counted, the better.